Photobehavior of Mixed nπ*/ππ* Triplets: Simultaneous Detection of the Two Transients, Solvent-Dependent Hydrogen Abstraction, and Reequilibration upon Protein Binding

Abstract

In the present work, a systematic study on hydrogen abstraction by the excited triplet states of 4-methoxybenzophenone (1) and 4,4'-dimethoxybenzophenone (2) from 1,4-cyclohexadiene (3), 4-methylphenol (4), 1,2,3,4-tetrahydroquinoline (5), and 1-methyl-1,2,3,4-tetrahydroquinoline (6) in different media has been undertaken. Laser flash photolysis (LFP) revealed that in nonpolar solvents, 1 and 2 triplets have a nπ* configuration with the typical benzophenone-like T-T absorption spectrum (λ(max) ca. 525 nm). Conversely, in aqueous solution, transient absorption spectra with maxima at 450 and 680 nm, attributed to the ππ* triplet, were obtained. Quenching of 1 or 2 triplet by 3 led to ketyl radical formation with rate constants in the range of 10(6)-10(8) M(-1) s(-1), being one order of magnitude higher in acetonitrile than in aqueous media. The rate constants of quenching by 4 and 5 were similar in both polar and nonpolar solvents; the highest value was found for 6 in acetonitrile ((6.3 to 6.9) × 10(9) M(-1) s(-1)). For mechanistic insight, LFP of 1 or 2 in the presence of dimethoxybenzene as electron donor was performed. The results showed that in this case, triplet quenching is favored in aqueous solution. In addition, 2 included in human serum albumin (HSA) was submitted to LFP. The decay kinetics, monitored at 430 nm, fitted well with three lifetimes of 0.45, 1.4, and 14.4 μs assignable to 2 in bulk solution and in site II or in site I of HSA, respectively. This assignment was confirmed by using oleic acid and ibuprofen as selective displacement probes.